I would buy it in a heartbeat! Here are some of my ideas:
Maybe the disc makers could inbed a microchip & we would find it using a cell phone.
Use a special ink in the plastic that can be seen with special glasses.
Something inbedded in the plastic that would conduct heat so we could find it with a infared device.
Something inbedded in the plastic that would conduct sound so you could track it by radio signal.
Something inbedded in the plastic that would emit a strong odor so we (or a dog) could track it by scent.
There's got to be a way!

All you would need is and RFID tag on the disc, and a handheld locator unit like you are talking about. The same way a lot of businesses track inventory. Here is a link to show you the different types of stick-on and implantable(inlay) sensors.

maybe it could also have a built in remotely controlled little balloon that filled with air on command to float it to the top?

and then maybe a miniature james bond could jump out and it would turn the disc into a jet-boat so he could drive it to shore and hand it to you. and then he could snap his fingers and make your favorite bond girl appear out of thin air to caddy for you and umm.. yeah....

I'm sure the technology is already developed using Nanotechnology...it just has to become affordable and available to the every day shmoe. A small fraction of a gram self adhesive microchip with GPS capabilities linked to a cell phone....piece of cake!!!!

Me and a friend were just discussing this today. They do make tracking chips small enough that you could just inject into the plastic. Kinda like those tracking devices you inject into pets. probably would be expensive though.